Relaxation of pulsed photocurrents under optical bias is studied in amorphous films of glassy alloys As₂Se₃:Sn_x (x=0 to 3.5 at.%) by the time-of-flight technique and steepness band-to-band light illumination. In frame of multiple-trapping model it is shown that by adding tin to the glass former As₂Se₃, the hole drift mobility is strongly increased and hamper the recombination. The kinetics of the long-term photocurrent decay can be described by stretched exponential function. The dispersion parameter α, which can be deduced from the time dependence of the photocurrent I_ph (t) ∞ exp[-(τ/τ) ^α] and, is about 0.47 for undoped samples, and 0.35 for tin-containing samples. The obtained results indicate the variation in occupation of deep localized centers. For the investigated samples, the width of distribution of the deep traps is approximately kT/α∼50-70 meV.